Variable-speed electrical machines

ABSTRACT

An arrangement to provide variation of the speed or other operating parameters of an electrical machine without connection to the rotor. The machine is of the induction type having a winding in separate parts to create a flux movable in the air-gap different speeds by the alteration of current in a part of the winding. In one example the current is altered by a variable impedance connected to one winding part, the other part being connected to an electrical supply. 
     Another example, FIG. 6, shows two machines (NM 1 , NM 2 ) arranged to drive a rail vehicle through smooth rail wheels attached to the machine rotor axles. A single, variable-frequency, inverter (NI) drives both machines and is connected to part (MP) of each machine winding. Other parts (AP) of each machine winding are connected together by a link (NC). In operation power (real or reactive) is transferred between the machines along the link to equalize the torque of the machines. Run-away of one machine while the other stalls is thereby avoided.

This invention relates to electrical machines which can be operated overa range of speeds.

Variable-speed electrical machines exist in various forms but in generalto achieve a variable speed a machine has to have a more complicatedconstruction, and often associated complex control circuits, than amachine for single speed operation. Examples include d.c. machines withcommutators and brush gear and field and/or armature current controlequipment and a.c. machines with complex inverters to provide reliablecommutation of s.c.r. devices under a range of operating conditions.Compound machines or machine sets, such as the Ward-Leonard arrangement,are also known. Other approaches include pole-switching, e.g. a 2/16pole induction motor, and the pole amplitude modulation technique. Thepole-switching technique while simple and robust has the penalty that alarger machine frame is needed as in effect the windings for both a 2pole and a 16 pole machine have to be accommodated.

With the present requirements to save both energy and materials there isa need for electrical machines which are economical in power andmaterial consumption while a simple and inexpensive variable speedcapability for a machine with the rugged construction of the inductionmotor type is also desirable. Generators with the ability to operatewell at a range of speeds are also in demand, particularly for wind orwater power drive.

Speed control for machines of the induction type, in practical terms, ispreferably at constant efficiency or constant torque and varies thesynchronous speed of the machine, that is the speed, V_(s), of theelectromagnetic wave along the air gap periphery where V_(s) =2T_(p) f.T_(p) is the pole pitch and f the supply frequency. Clearly the poleswitching technique varies T_(p). The variable-frequency inverter ormechanical commutator techniques vary f. The pole amplitude modulationtechnique varies the value of V_(s) by using a specific form of windingand external switches. Where some form of power conversion, e.g. aninverter, is used it is also possible to achieve speed control byvarying the voltage supplied to the machine.

It is an object of the invention to provide an improved electricalmachine arrangement for economic operation over a range of speeds.

In the Specification of UK Published Patent Application 2058478 andcorresponding Applications in other countries including U.S. Ser. No.,179,781, incorporated herein by reference, there is described anelectrical machine having a space-transient in the field conditions.Such a transient is there described as being provided in various waysincluding the use of some poles with shorter pitch than the remainder.The existence of the space-transient enables a motor form of the machineto recover energy from the rotor in a beneficial phase relation with theenergy supplied to the machine. The recovered energy can be applied toimprove the power factor or efficiency or other operating parameter ofthe machine, as described in the above mentioned specifications.

According to the present invention there is provided an electricalmachine arrangement having an air gap and a winding in separate parts tocreate in the air-gap, in operation, a machine air-gap moving fluxhaving a space-transient the arrangement including means to alter acurrent in a part of the winding to vary the effective speed of movementof the flux along the air-gap, whereby the adjustment providescontinuously variable control of at least one of the machine operatingparameters including speed, torque and efficiency.

Some of the separate parts of the winding may be connected to anelectrical power network and others of the separate parts to a variableimpedance.

Some of the separate parts of the winding may be connected to anelectrical power network in a specific phase arrangement and others ofthe separate parts connected thereto in a different phase arrangement toapply rotor induced voltage to said some winding parts in parallel witha power network voltage but with a phase variable in operation toproduce a self-compensating equal torque characteristic around thesynchronous speed.

A frequent arrangement of electrical machines is two or more machineshaving windings connected to an electrical power system and having amovable part of each machine coupled without a rigid connection. This isreferred to hereafter as multiple-operation of machines.

Examples of arrangements of two or more machines where the machines aremotors include vehicle traction systems where each motor drives one axleof the vehicle, as in a railway vehicle, and material conveyor ortreatment systems as in a steel rolling mill or paper mill. Thecouplings are respectively the rail, the steel billet and the paper webitself or a conveyor.

A serious problem of such systems, well-known in the art, is that theoverall operating conditions are never absolutely identical andtherefore each machine will provide a slightly different torque. Suchtorque differences will tend to increase the operating conditiondifferences in the absence of the rigid connection. The consequence in,for example, traction systems, is that the motor on one axle attempts tosupply all the tractive effort and is destroyed by overloading whileanother axle motor supplies none of the effort.

One known technique is to provide an individually controlled supply toeach motor. Again using the traction system example, each motor is aninduction motor and each is supplied with controlled frequencyalternating current from an a.c. or d.c. supply via a rectifier/inverteror inverter as appropriate. The frequency of the alternating current iscontrolled to prevent run-away of a motor by providing an equal torqueat each motor. Such a system is able to cope with most operatingconditions but involves a heavy penalty in additional equipmentincreasing capital cost and also increasing running costs by increasingmaintenance costs and tare weight which increases the power consumptionfor a given load.

It is an object of a particular aspect of the invention to provide animproved arrangement for multiply-operated electrical machines.

According to this aspect of the invention there is provided a machinearrangement for multiple-operation, as hereinbefore defined, includingat least two separate electrical machines each having a stator and arotor and individual mechanical load coupling means on the rotors for acommon mechanical load each machine having a stator winding including atleast a first and a second section and connections to each said section,the arrangement including a parallel electrical connection between afirst section of each machine and a series electrical connection betweensecond sections of each machine and an electrical supply networkconnection to said parallel connection, whereby in operation variationof the speed at the coupling to a common mechanical load is compensatedby current flow in the series connection equalizing the torque of themachines.

The machines may be traction motors driving a railway vehicle on a metalrail or rails by coupling the rails with smooth wheels on the axle ofeach machine rotor whereby machine speed variation with differentindividual axle driving conditions is compensated for. The first sectionmay be wound with a lower pole pitch than the second section.

The above-mentioned patent applications, and a paper by E. R. Laithwaiteand S. B. Kuznetsov read and published at the I.E.E.E. Winter PowerMeeting, New York, USA, February 1980, describe electrical machines forcontinuously generating reactive KVA which give improved machineperformance, e.g. better power factor and leading power factor in abrushless induction machine. Briefly the machines described are a.c.machines having a primary winding modified to create continuouslyoccurring transient electromagnetic conditions in distinction from thesteady dynamic conditions employed hitherto in induction and synchronousmachines. In one form for a cylindrical machine part of the stator isarranged to have a pole-pitch shorter than normal. For example a nominalten-pole machine is arranged to have poles as follows:

4-1*-4-1*.

1* indicates that more than a whole pole is provided in the spaceappropriate to one nominal pole. When the inductively coupled rotor,conveniently of squirrel-cage "solid" construction, passes from beneaththe 4 pole stator region to beneath the 1* pole region rotor current"memorised" from the 4 pole section causes current in 1* pole section.The current caused in the 1* pole section can be of unity or leadingpower factor. In use as a motor the 4 pole sections provide propulsionand the 1* pole sections permit recovery of power and control thequadrature flux component.

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 represents schematically a speed-controlled electrical machineaccording to the invention.

FIG. 2 shows typical speed characterisitcs for machines of the FIG. 1arrangement.

FIGS. 3 and 4 show graphs useful in understanding the speed/torquecontrol of the machines described herein.

FIG. 5 shows a traction motor speed control arrangement of the priorart.

FIG. 6 shows a traction motor speed control arrangement according to theinvention.

FIG. 7 shows one repeatable section of a winding layout for the motorsof FIG. 6.

FIG. 8 shows a naturally commutated induction motor drive embodying theinvention.

FIG. 9 shows an outline of power flow in the drive of FIG. 8.

FIG. 10 shows an arrangement embodying the invention in a link betweentwo power distribution networks.

FIGS. 11a,b,c shows various prior art a.c.--a.c. converter motor drivesystems.

FIG. 12 shows an arrangement of a naturally commutated motor with analternative start-up procedure.

FIG. 1 represents schematically an electrical machine according to theinvention wound on a conventional 12 pole cylindrical woundstator/squirrel cage rotor frame. The winding arrangement is actually2-1*-2-1*-2-1*-2-1* with 1* indicating the shorter pole pitch portions(as above). The winding is made so that one connection can be made tothe conventional poles as a group and another connection to the shorterpole pitch poles as a group. For convenience the conventional poles arereferred to as "mains excited" and the others as "rotor excited". It isto be noted that these terms do not limit the connection arrangementsand that the "rotor excited" poles are not connected to a rotor. Supplyconnections are not shown. Although shown as stator windings with theadvantage that connections to moving windings are not needed, it is madeclear that the use of these windings on the rotor is not excluded if itis convenient or appropriate to place them wholly or partly on therotor.

In one speed control configuration the "rotor excited" poles areconnected to a variable impedance Z external to the machine. By usingthis impedance to vary the current loading of these poles with respectto that in the "mains excited" poles the effective flux speed along theair-gap and therefore the slip and the speed of the machine can beadjusted over a significant range. This range can be about 25% ofnominal speed for machines of different "Goodness Factor G", asindicated in FIG. 2. J.sub.(SC) indicates the "rotor excited" polecurrent loading and J.sub.(MA) the "mains excited" pole current loading.ECM indicates the operating position for an "Equivalent ConventionalMachine" i.e. a 12 pole machine on this frame. The speed variation willbe at substantially constant torque while the rotor excess losses willnot be larger than the stator copper losses, the largest losses in themachine. If constant torque speed control is required the load profilemust be known so that the machine can be designed to suit this profile.

Although a wide range of values of the ratio of "mains" poles to "rotor"poles can be used the range of 3:1 to 2:1 is likely to give a goodpractical compromise between operating flexibility and efficiency. Ifthe number of "mains"/"rotor" poles sections is increased the range ofspeed at constant torque increases. If the number of sections is reducedthe rotor excess losses are reduced improving the efficiency.

There is of course some periodic variation of air-gap flux distribution.However if a reasonably high average value, say 0.75 Tesla r.m.s., ismaintained satisfactory operation at stable shaft speeds is achieved.

The presence of different currents in the "rotor" pole and "mains" polewindings creates a condition in which the slip is not constant aroundthe machine but has different values from point to point along the"mains" winding. Nonetheless the shaft speed quickly stabilizes inoperation at a definite and repeatable operating point. The differencebetween the currents in the "rotor" windings and "mains" windings causesa change in the in-phase (Bp) and quadrature (Bq) air-gap fluxcomponents under the "mains" entry and exit poles. This change willsatisfy the condition

    ∫Jrdθ=0

where Jr is the rotor current density and θ the angle around the rotorcage periphery. As mentioned above speed control at constant torque ismore effective with more numerous "mains"/"rotor" pole alternationswhile retaining a reasonable average air-gap flux density.

In another speed control configuration to vary the apparent synchronousspeed of one machine no external reactors are required. To achieve thevariation of current loading exerted by the "rotor" poles the phasing ofthe "rotor" poles is arranged to vary with respect to that of the"mains" poles. The induced voltage of the "rotor" poles should be justabout equal to the line voltage at the "mains" poles. The variation ofphase of the voltage induced in the "rotor" poles, with respect to thatin the "mains" poles, depends on slip approximately in a second-ordermanner. The graph in FIG. 3 shows a plot of phase angle between "rotor"and "mains" currents, quantity PRM, against slip on a per unit basis atconstant "mains" current. This graph is for a 50 Hz linear machine withthe "rotor" poles loaded for the purpose of the measurements by animpedance of (0.08+j2.13) ohms/phase but is representative of machinesembodying the invention operated at 50 Hz. It is an important feature ofthis aspect of the invention that the windings for both "mains" and"rotor" poles have a fixed number of turns and no tappings or switchconnections are needed. The link between the "rotor" and "mains" polesis passive and fixed, the phase change of the rotor induced voltagebringing about the control action. This arrangement is not specificallyshown in the drawings as it is produced by appropriate selection of thewindings for allocation between the "mains" and "rotor" poles, with thelink provided in the winding connections during manufacture.

In the machines described in the above-mentioned Applications, in whichpower factor is improved by regeneration of reactive voltamperes, theinduced voltage in the "rotor" poles must be significantly higher thanthe supply voltage, at least 20%, whereas for speed controlsubstantially equal voltages are required. Furthermore for speed controlthe variation of exit-edge (of the "mains" poles) loss voltamperes (EEL)with slip (s) should be as small as possible whereas for power factorimprovement the variation should be as sharp as possible with preferablya peak value between rated slip and synchronous speed, i.e. no slip.FIG. 4 shows a variation (A) more suitable for speed control, using 3poles in each "mains" group, while the other variations (B,C,D) are moresuitable for power factor improvement having a steep characteristic anda peak near to synchronous speed.

FIG. 5 shows in outline a known contemporary induction motor tractionsystem. Induction motors are attractive for traction purposes as theyare robust, without brush gear and commutators and can be provided withspeed control by supplying them from an a.c. or d.c. power source (P)via a variable frequency inverter. However when several axles of avehicle or train are provided with individual motors, as is the mostpractical arrangement for efficient construction and power economy, theneach motor M₁, M₂ must have its own control means and variable frequencyinverter I₁, I₂ to ensure that differences, e.g. in wheel size andtherefore axle speed, do not cause uneven division of traction effortbetween motors. This is a serious penalty and can in some cases lead tothe adoption of the conventional d.c. commutator motor and rheostaticcurrent control to avoid the complexities of multiple inverters andsupply frequencies f1, f2 while accepting the lower efficiency andhigher maintenance costs.

One aspect of the present invention applies to traction induction motorsand provides a solution to the torque division problem.

FIG. 6 shows a traction motor arrangement embodying the presentinvention. It will be seen that both machines NM₁, NM₂, or each machineif more than two, are supplied by one inverter, NI, which applies thesame frequency, f_(n), to each machine. The invention provides a meansof transferring power (which may be real or reactive) from one machineto another to compensate for potential torque differences and therebyequalize the torque from each machine despite axle speed differences.

In one specific form of the machine disclosed in the above Applicationsa conventional induction motor frame and rotor is provided with a statorpartly wound in one pole-pitch and partly in another. Generally thegreater part (the main part) MP is wound in a pole-pitch conventionallyappropriate to the intended use of the machine while a smaller part (theauxiliary part) AP is wound with a shorter pole-pitch or pitches. Forexample a conventional 10 pole frame could be wound with 4-1⁺ -4-1⁺poles, the 1⁺ indicating the shorter pole pitch with more than a wholepole in the space normally used for one pole. The dimensions for MP andAP in the figure are not necessarily those used in practice and arepurely diagrammatic.

The winding is made so that one connection can be made to theconventional "main" poles as a group and another connection to theshorter pole-pitch "auxiliary" poles as a group. In operation the "main"poles will generally be supply-excited and the "auxiliary" polesrotor-excited although this is not the only possible arrangement and isnot a limitation on possible arrangements within the scope of theinvention.

This aspect of the present invention provides that the "main" windingparts are connected in parallel to the inverter output while the two ormore "auxiliary" winding parts are connected together in a loop (NC) topermit the circulation of an equalizing current I₃.

In the prior art example to produce equal torque, T_(m), from eachmachine with a different axle speed S₁, S₂ the inverter outputfrequencies f1, f2 must be adjusted by a control action to equalize thetorque. In the arrangement embodying the invention the torque isequalized without any external control action by the flow of currentalong connection NC. The current, I₃, in connection NC is the product ofa constant, k, and the difference of the axle speeds S₁, S₂ ; that is

    I.sub.3 =k(S.sub.1 -S.sub.2).

The equalization current flow provides speed control without externalcontrol by electronic devices (signal or power types) by using the phaseof the rotor-induced voltage alone to control the maximum currentflowing to the main windings. Provided the load profile is known inadvance, as would be the case in traction and other multiple motorapplications, such as rolling mills, torque control can be achieved inthis way. The control loop can be considered as including thesupply-excited main pole windings and electrically-isolatedrotor-excited auxiliary pole windings of one machine magneticallycoupled through the common stator core and electrically connectedthrough the parallel supply connections and series auxiliary poleconnection to the other magnetically coupled machine windings. Thecontrol loop provides a self-compensating equal torque characteristicfor the connected machines, which may be more than two in number inwhich case all the auxiliary pole windings are connected in series.

In designing the windings the rotor-induced voltage should be aboutequal, say within one or two percent, to the supply line voltage. Alsothe exit-edge KVA loss, between the main and auxiliary poles should notvary significantly with slip. The "Goodness Factor" should be large;values in the hundreds are desirable. The number of poles in each mainpole group should be higher rather than lower. Thus in a 12 pole frame3-1⁺ -3-1⁺ -3-1⁺ is preferred to 2-1⁺ -2-1⁺ -2-1⁺ -2-1⁺ althoughundesirable unbalanced magnetic "pull" characteristics increase. The"3 - 1⁺ " arrangement can have a speed control range of 10% ofsynchronous speed, which should be more than adequate for torqueequalization. Excess rotor losses fall with main pole group size andvalues similar to core iron loss are attained for "3-1⁺ " so speedcontrol can take place effectively at constant efficiency. The lossesare therefore less than those with thyristor control and in a.c.commutator machines. The distinction from stepped speed-control achievedby pole-switching or switched pole amplitude modulation is emphasised asthe present invention provides continuous control, although in somecases over a limited range.

FIG. 7 shows one repeatable section and interconnections for twomachines driving separate traction axles via cage rotors. Each machineis on a conventional 10 pole frame and of 4-1⁺ -4-1⁺ type.

While described in terms of traction motors for individually drivenrailway vehicle axles the invention is also applicable to othermultiple-machine arrangements such as metal-working mills and materialconveyors where variation of speed of an individual drive can affecttorque equality.

In addition to the control of speed and/or torque to be substantiallyconstant under varying load or to be variable over a range there is aneed for traction purposes in particular to vary the operating speed ofa machine by varying the supply frequency. Techniques for achieving suchvariation using inverters, e.g. that shown in FIG. 5, are well known butthe problems associated with forced electronic or artificial commutationof the thyristor devices, using capacitors to provide the commutationenergy, are also well known and a drawback to the use of inverter-basedspeed control which is otherwise very attractive.

FIG. 11, at 11a, 11b, 11c, shows three known variable speed motorarrangements using an inverter in an a.c.-to-a.c. conversion at variablevoltage and variable frequency to operate an a.c. traction or similardrive motor from the a.c. mains to provide a variable speed drive. Ineach type the three-phase a.c. supply at a fixed frequency f1, say 50 or60 Hz, is changed to direct current at an adjustable voltage by asix-phase delay rectifier. An inductor is included in the connectionbetween the output of the rectifier and the input to the inverter.

In FIG. 11a the inverter is a line, or naturally, commutated currentsource inverter CSIA which uses six semi-conductor controlled rectifierdevices, e.g. silicon thyristors, to provide a three-phase a.c. outputat a variable frequency fv. Frequency fv is to be variable from zero toa value dependent on the number of poles of the motor and the requiredspeed but an upper value of 70 to 200 Hz covers most usual requirements.The variable frequency output of inverter CSIA is applied both to a cagerotor induction motor CRMA and to a synchronous condenser machine SCA.The synchronous condenser also requires a d.c. supply for the fieldwinding.

In FIG. 11b the inverter is a forced-commutated current source inverterCSIB which uses six main thyristors, six commutation thyristors and sixcommutation capacitors to supply a cage rotor induction motor CRMB.

In FIG. 11c the inverter is a line or naturally commutated, as in 11a,and is a current source inverter CSIC which supplies a variablefrequency and voltage to a synchronous motor SMC. The synchronous motoralso requiries a d.c. supply for the field winding or else, if this ispractical, a premanent magnet to provide this field.

Each of the above known arrangements has advantages and disadvantagesfor any specific use and power level. These are partly economic as therelative cost of the various components changes with power level andpartly technical as some techniques are appropriate to certain powerlevels only. Broadly the FIG. 11a arrangement is for powers of 10 MW andupwards, the FIG. 11b arrangement for powers of up to 100 kW and theFIG. 11c arrangement for powers of 100 kW to 10 MW with permanent magnetfields only possible at the lower power levels.

It will be clear that all the known arrangements require either theinverter or the machine(s) to be complex in order to achieve reliablecommutation. In particular when the machines are complex they require aseparate d.c. supply which greatly increase costs and operational workload.

FIG. 8 shows in outline an arrangement in which the inverter providingthe variable frequency for the machine does not require artificialcommutation. A suitable supply of electrical power PAC, typicallythree-phase 60 Hz mains at a convenient voltage, is applied to a phasedelay rectifier PDR to produce direct current at a controllablepotential. The direct current is applied, through a suitable inductor ifrequired, to a current source inverter, CSI. The output of the inverteris alternating current of a controllable frequency which is applied to aθ-pinch machine, TM, as described in the above-mentioned Applicationsand published paper. The "mains" and "rotor" poles (or motoring andasynchronous condenser poles) of the machine are both connected to theoutput of the inverter and the machine thus provides line-commutation ofthe inverter without the need for commutation capacitors, diodes etc.The d.c. link from the phase delay rectifier to the inverter preventsthe flow of reactive power so a reactive power balance must be achievedin the inverter and motor. The reactive (leading) power to commutate theinverter devices is provided by the "rotor" poles of the machine.

FIG. 9 illustrates the balance of real and reactive power when thearrangement of FIG. 8 is operating with natural commutation. Thereactive (leading) power from the "rotor" pole (condenser) windings,Q_(asc), supplies the reactive power to the motor, Q_(m), and thereactive power to commutate the inverter, Q_(inv). The real power flow(P_(m), P_(inv), P_(asc)) is also shown. The real power supplied to thecondenser windings (P_(asc)) is used for the additional stator copper(I² R) loss while the real power to the motor windings is used for motorstator copper loss, rotor copper loss and mechanical power. However thedirection of flow of P_(asc) can be controlled by the design of themachine and can be made zero or negative (at high slip) if required.

When the above described arrangements are compared with the prior artarrangements exemplified by FIG. 11 the following advantages are seen:

A.(a) No commutation capacitors and devices are required (e.g. FIG.11b).

A.(b) A separate synchronous machine is not required (e.g. FIG. 11c).

A.(c) Separate field excitation by direct current (FIGS. 11a and 11c),also requiring slip-rings, a permanent magnet (FIG. 11c), which is heavyand expensive, is not required.

A.(d) Harmonic currents, I₃ FIG. 8, are filtered into the asynchronouscondenser path instead of entering the torque-producing winding with thefundamental current, I₁ in FIG. 8, and affecting the smoothness of thedrive from the arrangement.

In operating an arrangement such as that exemplified in FIG. 8, thefollowing conditions are desirable:

B.(a) The value of Volts/Hertz at the motor should be substantiallyconstant as constant-slip operation is preferred.

B.(b) The phase delay rectifier PDR should be controlled to regulate thed.c. link to achieve the constant Volts/ Hertz ratio.

Such arrangements can produce a considerable range of running speeds athigh power levels. At the 100 kW level a speed range of 7:1 isattainable while at the level of 1 MW or more a speed range of 3:1 isattainable. The speed is continuously variable in the range. Therestriction of speed range at higher powers is not usually a limitationas a range of say 1,000 rpm to 3,000 rpm is more than enough to meetpractical requirements at the megawatt level.

The arrangements can also provide a braking mode with a regenerativeaction achieved by reversal of the voltage polarity in the d.c. link. Inpractice the characteristics of the mains supply, such as impedance, maylimit the effective range of regenerative braking.

A 150 h.p. 4-1⁺ -4-1⁺ pole machine has been built on a commercial 10pole frame and operated as a variable speed drive using an inverterhaving thyristors (International Rectifier type 101 RC 60) mounted onindividual heat sinks and controlled by gating signals generated in acontrol logic unit but without the use of commutation capacitors ordiodes.

In operation the machine runs at 10 pole speed. The machine iscontrolled to run always at a slip value appropriate to ensure adequatecommutation power. As with all commutation techniques a certain minimumtime must be allowed not a quantity expressed in electrical degrees.Therefore at low frequencies, e.g. below 15 Hz for 150 h.p., blanking ofthe d.c. link current by use of the phase delay rectifier in a pulsingmode is employed. This is the manner in which the machine is started andrun-up to the natural commutation speed, say 15 Hz, above which naturalcommutation is effective until the upper frequency limit is reached. Theupper frequency limit is reached when a safe commutating margin can nolonger be reached. For typical devices at the 150 h.p. machine level thecommutation time is 30 microseconds.

By consideration of the equivalent circuit of the machine at high speedand the accepted conventions on thyristor commutation a limitingfrequency of 200 Hz, at which the available commutation time, includinga 20 microsecond safety margin, of 50 microseconds is obtained. Anotherapproach is to consider the operating point at which unity power factoris reached for the machine.

For the machine whose details are given below at a slip of 0.075 thecommutation margin is adequate about 120 Hz. Operation up to about 200Hz is possible with a slip of 0.035. The machine is an 8 pole tractionmotor with about 31/2 poles of motoring winding in each of two repeatedsections. Other details are as follows:

    ______________________________________                                        Pole pitch (m)        0.194                                                   Stator slots          72                                                      Bor diam (m)          0.495                                                   Rotor bars            94                                                      Air gap (mm)          1.14                                                    Current density (A/mm.sup.2)                                                                        3.6                                                     Temperature rise (°C.)                                                                       75                                                      Max torque (Nm)       5010                                                    Current loading (kA/m)                                                                              48                                                      Rated slip            0.02                                                    Mechanical output at 100Hz (kW)                                                                     224                                                     ______________________________________                                    

The commutation limitations reflect the characteristics of currentlyavailable thyristors and not a fundamental property of the arrangements.As the turn-off time of the thyristors becomes shorter the power factorof the motor will approach unity, from the leading direction. Thyristorsfor the 100 kW to 10 MW power range at present require a commutationsafety margin of 20 microseconds and the induction motor then requires aleading power factor of 0.95 to 0.90 to operate at up to 200 Hz i.e.12,000 rpm for a 4 pole machine. In typical applications linecommutation at constant-torque, constant-slip and constant current isachievable for 15 Hz to 100 Hz i.e. 900 rpm to 6,000 rpm. However asthryristor characteristics improve the motor power factor can approachunity.

In start-up the phase delay rectifier is blanked at 60° intervals toproduce current commutation from the input side. At about 15 Hz inverterfrequency the effective value of the reactive output of the machine,which increases with frequency, is high enough to cause commutation andthis can then take over. The inverter requires about 11% of the reactivepower supplied to the "motoring" section (Q_(m) above) with a deviceturn-off time of 30 microseconds.

As described so far the arrangement has eliminated electrostatic storagedevices from the commutation process but still requires a speciallow-speed and start-up procedure. This can be overcome by the use of twoidentical PDR-d.c.-link-CSI chains with their inputs and outputs inparallel. In FIG. 12 by slowly increasing the current from eachrectifier, PDR1, PDR2, but in the opposite sense (polarity), for exampleusing ramp to give constant dIdc/dt of say 2A/ms the combined output ofthe inverters CSI1, CSI2 is a nearly perfect sinusoid at an appropriatevoltage-dependent low frequency, with a small component at twice powerfrequency, which is applied to a θ-pinch machine TMC. Despite the costand weight penalty of the extra 12 devices the arrangement is stillbetter than a capacitor/diode commutation arrangement. In a typicalarrangement the twice power frequency component is less than 5%. Thisarrangement overcomes a possible problem in some applications of thetorque pulsating resulting from the on-off current of the pulsingstart-up mode.

The θ-pinch machine can also provide a filter action to suppressunwanted current harmonics or incoming line transients. The filteraction is determined by the speed of the rotor which sets thefundamental frequency of the filter frequency response. The energy isabsorbed into the rotor where the cage construction can well withstandthe heating effect. For example the "rotor" pole (condenser) winding canbe parallel connected to the "mains" (motoring) pole windings and have areactance minimum at the fifth harmonic of the travelling wave tosuppress the dominant harmonic of the inverter which minimumautomatically "follows" the inverter frequency.

A further field of application of the invention is in the connection oftwo a.c. supply networks, such as those of separate public utilities orother large-scale generation and distribution systems, to permit powerflow. The frequency of such supply networks is not always synchronisedso interconnection is not practical. One solution is to connect via adirect current link which overcomes the problem of the difference infrequency. Another solution is to use a machine on each system andcouple the rotors mechanically and electrically. This requires woundrotors even if slip rings can be avoided by the use of two rotors on oneshaft. With suitable auxiliary plant to cause the rotors to rotate at aselected speed in one or other direction power can be transferred ineither direction. Even when both machines are assembled in one frameeach has to have the capacity to operate at the maximum power transferlevel. Also care has to be taken to avoid interaction between the twomachines (U.S. Pat No. 3,975,646 refers).

FIG. 10 shows an asynchronous power tie provided by a single cage rotorinduction machine (CRIM) having two stator sections according to thepresent invention. Power is transferred from a stator section of themachine which is connected to one power system (PN1) to another statorsection of the machine which is connected to the other system (PN2) viathe cage rotor. The cage rotor is driven by a suitable servo drivethrough a mechanical link indicated by the chain dotted line. The cagerotor is driven at a speed ω_(r) given by the expression:

    ω.sub.r =2ω.sub.1 -ω.sub.2.

Here ω₁ is the synchronous speed of network PN1 and ω₂ the synchronousspeed of network PN2.

When compared with the prior art system mentioned above there aresubstantial savings in both the main machine and the drive. The mainmachine is simpler, being of cage rotor not wound rotor construction andthe drive can be smaller for a given power transfer capacity of the tiepossibly as small as half the size for the prior art.

The arrangement described thus provides significant cost savings andsimplification.

The techniques described above provide a range of control on speed,torque and other operating characteristics of machines by making use ofelectromagnetic machine action based on reactive voltampere control.Clearly the techniques can be applied to generators of electricity aswell as to motors operated by electricity.

I claim:
 1. An alternating current electrical machine arrangementcomprising:a primary winding for connection to an alternating currentelectrical circuit of a predetermined frequency; a secondary windingmounted in a spaced relationship with and for relative motion withrespect to the primary winding an electrically isolated to define a fluxpermeable gap therebetween with relative motion between said primary andsecondary windings coupling them by electromagnetic effects; saidprimary and secondary windings formed of electrical conductorsdistributed in predetermined patterns along the extent of the fluxpermeable gap therebetween; at least one of said predetermined patternsof electrical conductors having a non-regular and non-uniform portionalong the flux permeable gap for creating and maintaining at least onetransient of electromagnetic field conditions along at least portions ofthe flux permeable gap; said at least one transient comprising a changefrom one effective field speed to another from point to point along theflux permeable gap; said at least one of said predetermined patterns ofconductors being provided by a winding including at least a first and afurther separate part; and means to alter a current in a said first partof a winding whereby at least one of said effective field speeds isaltered to vary the speed of the machine.
 2. An arrangement according toclaim 1 including a variable impedance as said means to alter current insaid first winding part and provide control of speed and means toconnect a said further separate winding part to an electrical powernetwork.
 3. An arrangement according to claim 1 including means toconnect a said further separate winding part to an electrical powernetwork in a specific phase arrangement and means for connecting saidfirst winding part to said network in a different phase arrangement andmeans to vary the phase to apply machine induced voltage to said furtherwinding parts in parallel with said power network voltage with a phasevariable in operation to produce a self-compensating equal torquecharacteristic around the synchronous speed.
 4. An electrical machinearrangement according to claim 1 including means for connecting separatewinding parts to separate a.c. power distribution networks to permitpower flow from one network to another through the machine arrangementdespite absence of synchronism between the networks.